Stereophonic to binaural conversion apparatus



May 7, 1963 B. B. BAUER 3,088,997

STEREOPHNIC TO BINAURAL CONVERSION APPARATUS Filed D90. 29, 1960 2 Sheets-Sheet 1 (SclNooasVmw) INVENTOR BENJAMIN B. BAUER WQBLMMB H IS ATTORNEYS May 7, 1963 B. B. BAUER 3,088,997

STEREOPHONIC TO BINAURAI.. CONVERSION APPARATUS Filed DeQ. 29, 1960 2 Sheets-Sheet 2 L STEREOPHONIC PROGRAM j INVENTOR BENJMI N B. BAUER Mig www H IS ATTORN EYS United s States 3,088,997 STEREOPHONIC T BINAURAL CONVERSION "APPARATUS Benjamin-B. Bauer; Stamford, Conn., assignor to Columbia Broadcasting System, Inc., New Y'rk,"N.Y., :i corporation lof NewYork FiledDec. '29, 1960, Ser.No.- 79,246 9 Claims. (Ci. 179-1) The present invention relates tio-sound reprodu'cingsys- "tems and, more particularly,l tost'ereophonic systemsof thisicharacter in: which means-is provided for 'reproducing f the sound binaurally in proper'space perspective so asto insure a high degree of naturalnessinthesound asheard by a listener.

BinauralV and stereophonic techniques haveibeen employed with-considerable success in Vsound 'recording and "reproduction to impart a sense of directiontto thefreproduced sound. In v"thef-binaural' method,` the sound -is picked up with -omnidirectional microphones vinstalled in a sphere oir-battle to-s'imulate'the ears ofavhuman'head and the signals-from the microphones are reproduced With corresponding earphones. In the stereophonic method, A'the'signals are picked up with highly directional microlphones or with spaced apart 'microphones and are reproduced with 'correspondinglspaced "apart"'loudspeakers. 'Where, however,i'program material lis'picked 'up or recorded by one of these'two techniques `and isreproduced "by the other, substantial'distortion of space perspective occurs. For example-'When program materialthat has I*been picked'lup orrecorded'stereophonically-is reproduced binaurally, as by'feeding Left and Right channel sig- `nalsv separately to each oftwo earphones,-the extreme Leftl and Right signals, respectively,=happear toV originate-directly youtsidethe listeners ears,'resulting ina gross distortion-'in space-perspective.

It is an object of the invention, accordingly, to provide a new and improved sound reproducing system'that is -lYfree'from the above-noted deficiency ofthe prior art.

Another object of the invention isl to provide new and improved sound reproducing Vsystems of the above charyfacter in which stereophonic program material can be reproduced binaurally without Isu'bstantial distortion `of space perspective.

These and other objects-are attained by causing binaural sound reproducing means terminating each channel of a stereophonic-sound reproducing systemto yrespond-both -to the program material transmitted by: the corresponding channel, -suitablymodified,` and yto the program material `ltransmitted by Vanother stereophonic `channel .of the system, appropriately delayed-intime.

1 the modification effected in each channelV simulates the More. particularly,

diffraction effect of the head of. av listener located inthe path of-sound from one -of the .loudspeakers in a stereophonic array, while the time delay in the program mateisV made to the following detailed description of several representative embodiments, taken in conjunction with the accompanying drawings, in which:

' FiG. l is a graph of typical relations that are helpful in understanding the invention;

FlG. 2 is a schematicv diagram of a system for reproducing stereophonic program material binaurallyaccordling to the invention; and

FIG. 3 illustrates schematically anotherembodiment of the invention.

ddee

Z -'Inf a-conventi'onal stereophonic sound system, sound -is f picked up' by-several-microphones placed near the 'performfers in a broadly-spaced array and suitably distributed l"between a-plurality of transmission channels usually two 5 in number. If'the program material is reproduced by a *plurality of loudspeakers distributed in space in approxi- Imately the same Way asfthe microphonesthefsoundA pattern produced at'the ears of allistener positioned in front of the loudspeakers will be similar to that produced at a cor-responding locationin front ofthe performers. 'In yother Words, each earof'the listener will receive sound Y pressure from each of the'loudspeakers, much'in the-same -mannerr as each ear Vof a listener infront of the lperform- 'erswillreceive sound pressure from directions both tol his i=left and. right.

When, however, the outputs of fLeftWand -fRighf 'stereophonic channels are' fed to earphones, respectively, Left:'and Right soundsfappearto originatedirectly #outsidef'theobservers ears, respectively, 'sof that algro'ss -'distortion in space perspective results.

Thisdistortion is Vlargly leliminated, according to' the f invention'bysuitablymodifying the signal in each chann'eland 'cross-feeding energy-between the channels so`as I .to simulate the sound pressure conditions prevailing at the'tears of a listener located in front of a pair of spaced loudspeakers connected to. receive the outputs of the f stereophonic channels. For a listener location at a position making an angle of, say, 45 with a' pair of speakers r in a stereophonicarray, it'can'be shown vthat the ksound 3()Y pressures producedv by the .left speaker at the left and t. 'rightv ears 'ofa listener vary with frequency relative to "the sound pressure for a head-on situation approximately in the manner'indicated by'the dotted line-'curves 10 andi 11, respectively, in FIG. l. rlhese Tcurves are ybased on data reported in apaper `by F. MfWeinerentitled On'the Diffraction of a ProgressiveSound Wave `by the YHuman Head, The Jour. ASA, 19;v 143-146, January `l947. Similar curves obtain forthe sound pressures produced lby the rightspeakerV ofthe array, except 40-thatthe curves land- 11inthis case correspond tothe relative pressures lat the right and leftv ears,respectively. VIt"willibe--app'arent that the sound from each off two speakers-disposed'in a stereophonic array does not :arrive --simulta'neously at the ears of a listener located between fand'in fro'nt-of-them. On the contrary, there' is a time -delay resultingfrom1the 'fact'that-the-ears are'lspaced apart in-:the direction of propagation-ofthe sound. For a listener :located at the 'intersection of `twolines 'each -making anangle ofv 45 c"with a line through the two speak- -fersinfthe stereophonic array '(a-:preferred location for fstereophonic listening), the'time delay between theleft .-.and-.right ears of-the listener can'bevtaken to 'be .4 millisecond, corresponding toa projected interaural 'distance -f of 5.5- inches lon the 45 line to oneof thefspeakers.

Correctreproduction of stereophonicV program material binaurally, therefore,` can ybe achieved-by modifying the signal from` the Left channeLwhich is to energizethe Left binaural sound reproducer, "in -the -rnanner indi- "cated by the/curve 10Min FIG. 1; y'combining'with it: a 60. signal 'derived from the Right channel, modified as-indicated =by` the curve 11 in FIG. 1, and delayed in time'apf proXirna-tely '.4-"secondg and'etfectingl a comparable'niodicationlof'thesignalfrom the Right channel with 'cor- 5- responding-cross-feed' Iof energy from the Left channel. `VActually, only frequencies vbelow'about onel kilocycle need beso delayedsince, atffrequencies above this value, stereophonic perception appears to be a function largely vof sound intensity.

A symmetrical network T capable yof modifying stereo- I phonic signals essentially in =the manner described above is shown irrFIG.Y 2. It comprises a tirs-t'series arm including a resistor 12 and an inductor 13 in series, a second series arm with a resistor 14 and an inductor 15, and a shunt arm including a capacitor 16 connected in shunt with a series combination including a resistor 17, an inductor 18 and a capacitor 19. The network deiines a pair of signal .transmission channels, one of which is adapted -to receive at its input terminals 20 a rst stereophonic signal EL from a source 21 and to provide a binaural output at its output terminals 22, and the other of which has input terminals 23 adapted to receive a second stereophonic signal ER from a source 24 and output yterminals 25 at which a second binaural output appears. The :output terminals 22 and 25 of the two transmission channels are connected to suitable binaural sound reproducing apparatus such as a pair of earphones 261J and 26B, respectively. As indicated by the arrows in FIG. 2, the signals EL and ER are connected in a series additive sense, and the polarity of the earphones is likewise in a series additive sense as indicated by the dots on the earphones 261J and 26B.

In a practical system for operation from low imped- 'ance program sources (say, 4 ohms or less) and to feed high impedance earphones (say, 2,000 ohms or more), ythe resistors 18 and 20 may be 60 ohms each, Ithe inductors 19 and 21 8 millihenries each, the capacitors 23 and 26 3.() microfarads and 3.8 microfarads, respectively, the 'resistor 24 30 ohms and the inductor 25 4 millihenries.

The solid curves 27, 28 and 29 in FIG. l illustrate the performance of the system shown in FIG. 2. The curve 29 indicates that .the desired delay time is well achieved up to 1 kilocycle per second, which is the frequency range where delay is most effective. Above l kilocycle per second, the amplitude function is more important (curves 27 and 2S) and it is seen that these curves approximate the .theoretical curves 10 and I11 reasonably well. By making the network components variable, an adjustment f the desired action may be obtained.

In operation, the left channel of the network in FIG. 2 supplies to the left earphone 261, an output signal that is a composite of the stereophonic signal EL, modied in frequency response as indicated by the curve 27 in FIG. l, and the stereophonic signal ER, modied in frequency response as indicated by the curve 2:8 in FIG. 1 and delayed about .4 second in time (at least for frequencies up to about 1 kilocycle per second). Similarly, the right channel supplies to the right earphone 26E an output signal that is a composite of the stereophonic signal ER, modified in frequency response as indicated by the curve 27 in FIG. 1, and the stereophonic signal EL, modied in frequency response as indicated by .the curve Z8 in FIG. l and also delayed about .4 second in time for frequencies below about l kilocycle per second. Accordingly, the sound patterns at the ears of a listener wearing the earphones 261l and 26B will approximate those obtaining with lthe listener located in front of a pair of loudspeakers energized in response to the stereophonic signals EL and ER, respectively.

It will be noted that the network not only provides .a cross-feed with suitable frequency response and time ydelay characteristics, but also modifies the corresponding signal in a manner similar to that which is caused by the diiraction by the head of the listener. Also, with two identical Left and Right signals, the network of FIG. 2 will provide a flat transmission characteristic, correponding to the 0 incidence response in FIG. 1.

yFor low impedance earphones (i.e., ohms or less), it is more convenient to use the system shown in FIG. 3, which is the dual of that depicted in FIG. 2. Here, the earphones as well as ythe stereophonic program sources are connected in a parallel -additive mode. In FIG. 3, the program source 21 supplies fa signal EL through a resistor 30 Ato a shunt network arm including the earphone 261, and a parallel circuit including a capacitor 31 and a resistor 32. Similarly, the program source 24 supplies a signal ER through a. resistor 33 to another shunt network arm including the earphone 26R' and a parallel circut including a capacitor 34 and a resistor 35. The two shunt arms containing the earphones 261, and 26R are connected together by a conductor 36 and by a coupling including an inductor 37 in series with a parallel combination including a resistor 38, a capacitor 39 and an inductor 40.

In a practical case, the resistors 30 and 33 may be 300 ohms each; the resistors 32 and 35 50t ohms each; the capacitors 31 and 32 2.7 microfarads each; the inductors 37 and 40 9 millihenries and 11.4 millihenries, respectively; the resistor 38 100 ohms; and the capacitor 39 `1.3 microfarads. The system response is essentially as depicted =by the curves 27, 28 and 29 in FIG. l.

The invention thus provides novel and highly effective means for reproducing stereophonic program material binaurally. `By modifying each of the signals fed to the reproducers in the manner described above, the correct sound pattern may be produced at the ears of the listener for reproduction without distortion in phase perspective.

The several embodiments described above are intended to be merely illustrative and are susceptible of modifica- 4tion in form and detail within the spirit of the invention. yFor example, acoustical tubes with appropriate delay characteristics may be used instead of electronic circuits for modifying the sound pressure at one earphone of a binaural system as a function of the sound pressure at the other. Also, while the networks herein described provide .a =most eicacious means for converting a stereophonic program into a binaural program, a less complete network may also be used. For example, a network providing a frequency discrimination only, without delay as shown by curves 27 and 28 in FIG. 1, or a delay only .as shown by the curve 29 in FIG. l, can be used with similar effect. Moreover, it will be understood that the specific values of electrical constants mentioned above are merely exemplary and other values can be used within the skill of the art. The invention, therefore, is not to be limited to the specic structures described and illustrated herein but embraces all modifications thereof coming wi-thin the scope of the following claims.

I claim:

l. In a sound reproducing system, the combination of a pair of earphones, a pair of signal channels connected to energize said respective earphones, and means for effecting additive transfer with time delay of signal energy from each of said signal channels to the other, said time delay being of the order of the time required for sound to travel a distance equal to Ithe distance between the ears of a listener.

2. In apparatus for reproducing stereophonically related signals binaurally, the combination of a pair of signal channels adapted to receive a pair of stereophonically related signals, respectively, and to provide a pair of binaurally related output signals, respectively, and means in each of said channels for modifying at least a part of the stereophonic signal received thereby and for eifecting cross-feed of stereophonic information therebetween in accordance with a given time delay characteristic approximating the time .required for sound to travel a distance equal to the distance between the ears of a listener.

3. Apparatus as in claim 2 for reproducing stereophonically related signals binaurally in which the stereophonic information time delay characteristic simulates the interaural time delay for a listener with respect to one of a pair of speakers disposed in a stereophonic array.

4. Apparatus las in claim 2 for reproducing stereophonically related signals binaurally `in which the modification eifected by the modifying means in each of the channels simulates the modiiication of sound pressure generated by a loudspeaker caused by dilfraction by the head of a listener.

5. Apparatus as in claim 2 for reproducing stereophonically related signals binaurally in which the modifying means in each of the channels effects a rise in frequency response above about 200 cycles per second.

6. Apparatus as in claim 2 for reproducing stereophonically related signals binaurally in which the modication elected by the modifying means in each of the channels simulates the modification of sound pressure generated by a loudspeaker caused by diffraction by the head of la listener, and the stereophonic information time delay characteristic simulates the interaural time delay for la listener with respect to one of a pair of speakers disposed in a stereophonic array.

7. In a sound reproducing system, the combination of a symmetrical T network having adjacent iirst and second series arms and a shunt arm therebetween, resistance and inductance means in each of said series arms, a pair of sound reproducer means shunting said first and second series arms, respectively, resistance, inductance and capacitance means in said shunt arm, capacitance means connected in parallel with said shunt arm, first network input means connected to a terminal of one of said series arms and to a terminal of said shunt arm, and second network linput means `connected to a terminal of the -other of said series arms 4and to said terminal of the shunt arm.

8. In a sound reproducing system, the combination of a network having rst, second and third series arms and iirst and second shunt arms between said first and second and said second and third series tarms, respectively, resistance means in said first and third series arms, respectively, sound reproducer means in series with resistance and induotance in parallel in said iirst and second shunt arms, inductance means in series with resistance means, capacitance means and inductance means in parallel in said second series arm, rst network input means including said iirst series arm and said iirst shunt arm, and second network input means including said third series arm and said second shunt arm.

9. Appanatus as in claim 2 for reproducing stereophonically related signals binaurally in which the modifyirrg means is variable so as to enable adjustment of the modiiication effected thereby.

References Cited in the tile of this patent UNITED STATES PATENTS 2,819,342 Becker Jan. 7, 1958 2,836,662 Vanderlyn May 27, 1958 2,845,491 Bertram July 29, 1958 OTHER REFERENCES Radio land TV News, publication by Feldman, March 1959, page 71. 

1. IN A SOUND REPRODUCING SYSTEM, THE COMBINATION OF A PAIR OF EARPHONES, A PAIR OF SIGNAL CHANNELS CONNECTED TO ENERGIZE SAID RESPECTIVE EARPHONES, AND MEANS FOR EFFECTING ADDITIVE TRANSFER WITH TIME DELAY OF SIGNAL ENERGY FROM EACH OF SAID SIGNAL CHANNELS TO THE OTHER, SAID TIME DELAY BEING OF THE ORDER OF THE TIME REQUIRED FOR SOUND TO TRAVEL A DISTANCE EQUAL TO THE DISTANCE BETWEEN THE EARS OF A LISTENER. 